Microporous polyolefin membranes are widely used in separators for lithium secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries and polymer batteries as well as electrolytic capacitor separators, various filters such as reverse osmosis membranes, ultrafiltration membranes and microfiltration membranes, steam-permeable, waterproof clothing, medical materials, etc. When microporous polyolefin membranes are used as battery separators, particularly as lithium ion battery separators, their performance largely affects the performance, productivity and safety of batteries. Accordingly, microporous polyolefin membranes are required to have excellent permeability, mechanical properties, heat shrinkage resistance, shutdown properties, meltdown properties, etc.
In general, microporous membranes made only of polyethylene have low meltdown temperatures, while microporous membranes made only of polypropylene have high shutdown temperatures. Thus, microporous membranes based on polyethylene and polypropylene are suitable for battery separators. Thus proposed are microporous membranes made of a mixture of polyethylene and polypropylene, and multi-layer, microporous membranes comprising a polyethylene layer and a polypropylene layer.
For instance, JP 05-251069 A and JP 05-251070 A disclose a separator free from thermal runaway, which is formed by a multi-layer, microporous sheet comprising a first layer made of an ethylene-butene copolymer or an ethylene-hexene copolymer for causing shutdown at 80 to 150° C., and a second layer made of polypropylene for causing shutdown at a higher temperature by 10° C. or more than in the first layer.
JP 05-251069 A discloses a method for producing a multi-layer, microporous sheet comprising the steps of producing a laminate sheet comprising a layer made of the above copolymer and an extractable solvent and a layer made of polypropylene and an extractable solvent, removing the extractable solvent from the sheet to obtain a microporous sheet, and stretching the microporous sheet at a temperature of 25 to 110° C. JP 05-251070 A discloses a method for producing a multi-layer, microporous sheet comprising the steps of simultaneously extruding a melt of the above copolymer and a polypropylene melt, cooling the melts to obtain a laminate sheet, stretching the laminate sheet at a temperature from −198° C. to −70° C., and heat-treating the laminate sheet.
However, investigation by the inventors have revealed that the methods described in the above references produce multi-layer, microporous membranes comprising a polypropylene layer and a polyethylene layer, which have small pore diameters in the polypropylene layer, thereby having insufficient permeability. Thus desired is a multi-layer, microporous polyolefin membrane comprising a polyethylene layer and a polypropylene layer, thereby having well-balanced permeability, mechanical strength, heat shrinkage resistance, shutdown properties and meltdown properties.